Microorganisms such as algae, archaea, bacteria and fungi including filamentous fungi, mold and yeast may contain triglycerides up to 80% of their total dry matter content. However, oil from microbial biomass which is suitable as precursor for fuel production is scarce on the market. This is mainly due to lack of efficient and economical methods for providing good quality oil from microbial biomass.
The available methods for extracting oil or lipids from microbial biomass typically require the biomass to be dried and/or microbial cells to be disrupted. Drying of the biomass consumes much energy and is, for example, performed after centrifugation by contact drying, spray drying or even by freeze drying. The typical water content or the dry matter content of the biomass is dependent on the microbial material used. Typically dry matter contents from 15 up to 40% by weight can be achieved by traditional cell harvesting techniques such as centrifugation or filtration. Essentially, it is traditionally aimed at as low free water content as possible in order to maximize the extraction yields.
One alternative method for acquiring oil from biomass is to apply non-selective extractants which typically produces oil containing high amounts of impurities. Impurities such as metals, phosphorus and amino acids cause problems e.g. in the fuel production in form of catalyst poisons and/or corrosive materials. Therefore, it is often required to use post processing for removal of these undesired components from the extracted oil product.
Especially, the high amount of phospholipids in microbial biomass, i.e. the amount of membrane lipids from the total lipid content, is problematic. These phospholipids are typically in the form of metal salts providing high metal content into oil. Traditionally, these phospholipids have been removed from the crude bio oil fraction before further processing such as refining with catalytical processes.
In general, methods available suffer either from lack of selectivity to produce good quality oil or poor yield which are compensated by additional processing steps or selection of uneconomical processing conditions.
US2007218175 discloses a method for extracting an oil bearing plant with fatty acids alkyl esters at temperatures from 15° C. to 180° C. The inventors report that generally better yields are obtained with higher temperatures, but on the other hand, higher temperatures result in oil products with higher amounts of phosphorous. The use of higher extraction temperatures is thus not considered advantageous and extraction of wet microbial biomass is not disclosed.
U.S. Pat. No. 4,857,329 discloses an extraction method where fungi is extracted using a solvent in a supercritical state or a mixture of a solvent in supercritical state and a cosolvent selected from butane, pentane, hexane, heptane and cyclohexane. The pressures applied in order to provide the supercritical state are in the range of 200-600 kg/cm2. The fungi cells are dehydrated to a moisture content of 50-70%, heated to temperatures of 150-200° C. and extracted with at least the solvent in supercritical state at temperatures below 90° C. There is no analysis given on impurities contained in the obtained oil product.
WO2008034109 discloses a method for recovering fatty acids in form of alkyl esters from microbial biomass, such as microalgae, bacteria and fungi. The biomass in treated at high temperatures up to 450° C. and elevated pressure, such as up to 40 MPa (about 400 bar). This high temperature treatment aims at and results in disruption of the cells and formation of an oily phase. An alcohol, such as methanol or ethanol, is added to the oily phase and alkyl esters (FAME or FAEE) are formed. Co-solvents, such as alkanes, and catalyst, such as organic acids, can be used. Esterification reactions require essentially water free environment and a high amount of alcohol present.